The invention concerns a method for charging an actively shielded magnet coil arrangement, which is superconductingly short-circuited during operation via a superconducting main switch, in particular, of an NMR magnet, wherein the magnet coil arrangement comprises at least one first and one second partial region, wherein at least the first partial region can be separately short-circuited via an additional superconducting switch, and wherein the two partial regions generate magnetic fields during operation whose magnetic dipole moments D(1) and D(2) have opposite signs.
Actively shielded magnets have a considerably reduced magnetic fringe field outside of the magnet coil arrangement, i.e. the field strength decreases rapidly with increasing separation from the magnetic center of the magnet compared with an unshielded magnet of the same field strength. This reduced fringe field provides the following substantial advantages for the user.    1. For safety reasons, magnetic fringe fields of a certain magnitude (5 Gauss=0.5 mT) must remain within a spatial region which is inaccessible to the public. Shielded magnets considerably reduce this spatial region.    2. In addition to NMR, other measuring methods are often used in a laboratory, which are sensitive to magnetic fields (e.g. mass spectroscopy). The combination of differing measuring methods within a common, confined region, may, in some cases, only be possible using shielded magnets.    3. The number of NMR magnets within a laboratory can be considerably increased by using shielded magnets.
The planning of buildings and utilization of space is therefore much simpler and effective for actively shielded magnets compared to magnets without shielding. Magnets of a certain field strength can often only be used in a given area when the fringe field is actively shielded.
The fringe field specifications must be met for each magnet, irrespective of tolerances in the production of the magnet and of the superconducting wires. Dimensional variations in the superconducting wires change e.g. the current density in the magnet coils. These tolerances can have direct and indirect influence on the fringe field. Direct influence, in that the field portions of the shielding magnet region and of the residual magnet do not correspond to the optimum theoretical values due to tolerances, which results in deterioration of the fringe field compensation. Indirect influence, in that field inhomogeneities are generated at the NMR sample (i.e. in the magnet center) due to production tolerances, whose correction requires so-called cryoshims which, in turn, generate a fringe field which is superimposed upon that of the magnet. Both influences are substantially caused by production tolerances and therefore differ for each magnet system. To solve this problem, the specified fringe field spatial region of the magnets could be increased to such an extent that the admissible values are not surpassed despite all production tolerances. Characteristic separations are e.g. the radial and axial separations of the 5 Gauss line from the magnetic center of the coil arrangement. This procedure is, of course, greatly disadvantageous in that the theoretically possible fringe field reduction of the magnet is not utilized. The present patent provides a very simple and elegant method which permits utilization of the theoretical potential of fringe field reduction. The method permits compensation of the fringe field during charging and the correction can be adjusted to each individual magnet.
Formal description of active shielding:
The magnetic far-field region of any magnet arrangement always has a dipole character at a sufficiently large separation from the magnetic center, since the far-field is dominated by the dipole term of the multipole expansion of the magnetic field. To reduce the fringe field or the far-field of a magnet, the dipole moment of the magnet arrangement must be largely eliminated. Towards this end, an unshielded main magnet coil is augmented by a shielding section which is connected in series and is wound in an opposite sense. For an optimized, i.e. minimized fringe field, the dipole moments (D, [D]=A*m2) of the shielding section (A) and the main magnet coil (H) must be approximately equal and opposite, i.e. the following must apply:DA+DH≅0. 
In this case, the fringe field is substantially determined by terms of higher order. The dipole moment of a magnet coil can be described as a product between a pure geometrical value and the coil current. The value which depends on the magnet geometry is referred to below as the dipole moment lift d (d=ΔD/ΔI, [d]=[D/I]=m2). The dipole moment of the magnet coil arrangement through which a current IMagnet flows, can be represented as follows:D=(dA+dH)*IMagnet 
The dipole moment lift is defined to include the winding sense of the magnetic coil and can therefore also be negative.
An additional dipole moment (D*) of a theoretically fringe-field optimized, actively shielded magnet coil arrangement results e.g. from a deviation of the dipole moment lift from the theoretical value in consequence of production tolerances. As mentioned above, charging of the cryoshim can also influence the fringe field distribution. This error may formally also be associated with an effective dipole moment lift of the overall arrangement (magnet coils+cryoshim system) for which reason the two causes are no longer distinguished below.
It is the object of the present invention to compensate, during charging of the magnet, for the additional dipole moment which is caused through production tolerances and which is different for each magnet to such an extent that the theoretically achievable minimum fringe field can be obtained during operation. The fringe field region can thereby be specified within narrower limits thereby considerably facilitating the planning of buildings and utilization of space for NMR magnets or even enabling their use in environments with limited space.